In the majority of railway networks, the railway is constituted by rails fixed on ties, or sleepers, placed perpendicularly to the rail, said ties resting on a thickness of stones called ballast. This traditional design of the track has been very successful and has proved perfectly adaptable to varied types of rail traffic.
However, cases exist where another railway technique is preferred to this traditional track, in particular when the supply, transport or maintenance of the ballast and of the track proves problematic: this is the case in particular in a zone of difficult access, such as in a tunnel. In that case, a so-called unballasted track is used, whose bed is made of concrete which replaces the bed of stones.
A particularly wide-spread embodiment of unballasted track comprises, as shown in FIG. 1, a cross-tie constituted by two blocks of concrete or tie-pieces, connected together by an angle piece called brace. The shape of the lower part of the concrete blocks is designed so as to perfectly match the shape of an enveloping shell made of elastomer and called "slipper".
Between the lower face of the concrete block and the bottom of the envelope is generally disposed an elastomer sole whose surface is substantially equal to the surface of the lower face of the block. This sole presents a suppleness adapted to the nature of the traffic on the line and gives the track an elasticity in the vertical direction which replaces or improves the elasticity provided by the conventional ballast.
The lateral walls of the envelope are provided with relief elements of the groove type which ensure a certain elasticity in the horizontal plane. In the upper part of the envelope, a bead, disposed over the whole contour, allows a correct hold of the envelope on the blocks of concrete, avoiding excessive gaping.
This bead, combined with a particular geometry of the blocks of concrete, limits the possible introduction of water between the envelope and the block.
When the track is laid, the assembly constituted by the rails and ties joined together by a fastening system, is positioned with the aid of wedging devices and a so-called wedging mortar is poured, which fixes the track in a geometrical position determined once and for all. The mortar is poured up to a height slightly lower than the level of the lower edge of the bead of the shell; the lower part of the envelope is then surrounded with mortar, forming a housing in honeycomb form.
This type of unballasted track is mainly used for applications in tunnels, for underground networks or for conventional lines of railway networks where the trains circulate at speeds below the high-speed domain of which the lower threshold may be situated at approximately 200 km/hr.
High-speed tracks pose, in fact, a particular problem which resides in the fact that the dynamic movements of the rail and of the tie upon passage of the convoys are substantially greater than on a track intended for circulation at normal speed; now, with reference to FIG. 1, it appears that nothing limits the upward displacement of the rail and tie except the gravity exerted on these elements. It follows that a lifting of the blocks and the rail generated by track deformation waves cannot be excluded.
Such lifting may prove critical if its amplitude is such that a clearance may occur between the block and the sole or between the sole and the bottom of the envelope; in that case, a shock may result during the descending movement of the block when the clearance is cancelled.